Macrocyclic alkenes having 8 to 20 carbon atoms can be used as intermediates for the preparation of fragrances. 14- to 17-membered cycloalkadienes are used in particular as starting materials for the preparation of musk fragrances. Cyclohexadecenone can be prepared from cyclohexadeca-1,9-diene (S. Warwel, H. Bachem, N. Doring, H. Katker, E. Rose in Seife-Ole-Fette-Wachse 115, 538 (1989)). In general, it is necessary for the starting compounds to be virtually free from impurities.
Cycloalkadienes are usually obtained by metathesis of corres-ponding cyclomonoenes. By way of example, metatheses, which may be mentioned are those described in U.S. Pat. No. 3,935,270, Brit. Pat. No. 1,105,565, Brit. Pat. No. 1,118,517, EP 182 333, and also Warwel, H. Ridder, G. Hachen in Chemiker-Ztg. 107, 115 (1983).
A disadvantage of these processes is the large amount of solvent, which has to be used to ensure high selectivity for desired cycloal-kadienes. The known isolation processes are saddled with high-energy costs and are therefore expensive. Isolation is usually carried out by means of distillation.
The metathesis of cyclooctene or cyclopolyoctenylenes to give cyclohexadecadiene is carried out as described, for example, in EP 182 333 and EP 343 437. In the liquid phase, the mixture of starting materials is brought into contact with the heterogeneous catalyst system Re.sub.2 O.sub.7 /gamma-Al.sub.2 O.sub.3 in the presence of a tetraalkyltin.
The reaction is preferably carried out at temperatures of from 20 to 60.degree. C. and using from 0.01 to 0.05 molar solutions. The contact time on the catalyst is typically from 25 to 200 seconds. The molar concentration of the solutions, which is given, refers to the calculated cyclomonoene units, which result from the division of the cyclopolyenes into monomers.
In the process described above, cyclooctene or a cyclooctene/cyclopolyoctenylene mixture produces a reaction mixture, which essentially comprises cyclohexadecadiene, cyclotetracosatriene, cyclodotriacontatetraene, cyclotetracontapentaene and cyclooctatetracontahexaene. The proportion of cyclohexadecadiene is generally in the range from 20 to 50%, preferably from 25 to 35%.
In the same process, cyclooctene and cycloheptene produce a reaction mixture, which comprises cyclotetradeca-1,8-diene, cyclopenta-1,8-diene and cyclohexadeca-1,9-diene, and higher macrocyclic cyclopolyenes. The proportion of dienes is generally in the range from 20 to 50% preferably from 25 to 40%.
The reaction mixture is usually in the form of a solution in metathesis inert solvents.
In the above-process, the preferred solvents here are unbranched or cyclic hydrocarbons. The unbranched are compounds having from 5 to 12 carbon atoms, preferably from 5 to 8 carbon atoms, for example n-pentane and n-hexane. The cyclic hydrocarbons are compounds having from 5 to 8 carbon atoms, preferably from 5 to 6 carbon atoms, for example cyclohexane.
It is known that crystalline aluminum silicates (zeolites) can be used for separating hydrocarbons. Thus, for example, U.S. Pat. No. 3,668,730 describes large-pored zeolites for separating xylenes. In U.S. Pat. No. 4,313,014, such zeolites are used for separating off cyclohexenes.